The shortest-path-first (SPF) routing method is used in a variety of data networks to determine the path data should be routed from any source node to any destination node in the network. Examples of data networks that use the SPF routing method are Internet Protocol (IP) networks using OSPF or IS-IS versions of the Interior Gateway Protocol (IGP).
The nodes in an SPF-routed network are connected by unidirectional links. Each link is assigned a positive number called a metric. The length of any path through the network is defined to be the sum of the metrics on the links comprising that path. Data traveling from a source node to a destination node travels along the shortest path, according to this definition, between these two nodes. If more than one path is of equal cost, the data load, or bandwidth, is shared in some manner between these equal-cost paths.
The choice of network metrics, then, determines the routing of data through the network. Certain choices of routings through a given network may be more desirable than others. Metric-based traffic engineering is the process of choosing metrics to achieve desirable routings. Certain routings may be preferred over others for any of a number of reasons, for example:    1. To keep the bandwidth utilization (e.g., the percentage of link capacity utilized) below a certain percentage, or as low as possible, for each link in the network, to prevent congestion.    2. To keep bandwidth utilization below a certain percentage, or as low as possible, for each link in the network, even when certain elements of the network (links or nodes, for example) fail, or are purposefully deactivated, and bandwidth must be routed elsewhere through the network.
Two aspects of this field have been the subjects of some prior academic research:    1. Estimation of point-to-point bandwidth demands in a network from measurements of link utilizations. See, for example, “An Information-Theoretic Approach to Traffic Matrix Estimation”, by Yin Zhang, Matthew Roughen, Carsten Lund, and David Donoho, ACM SIGCOMM 2003. Knowledge of the point-to-point demands is necessary to calculate future link utilizations under modifications to the metrics and/or the topology of the network.    2. Algorithms for selecting metrics in a network to avoid congestion through over utilization of links under normal network operation. See, for example, Traffic engineering with traditional IP routing protocols, IEEE Communications Magazine, 40(10): 118-124, 2002.